In the conducting airways of the respiratory system, the mucociliary system serves as the primary defense mechanism to move inhaled particles or infectious agents out of the airways in the lungs. In addition, substances present in airway fluids serve to limit the toxicity of the partides and to inactivate infective agents. The physical mechanism of coughing serves to expel the mucus from the airway passages (see e.g., “Foundations of Respiratory Care” Pierson and Kacmarek, eds. (1992) Churchill Livingstone Inc. New York, N.Y.; “Harrison's Principles of Internal Medicine”, Fauci et al., eds. (1997) 14th Edition, McGraw Hill, New York, N.Y.).
The mucociliary system consists of ciliated epithelial cells, epithelial goblet cells, and serous and mucous cells located in submucosal glands. The cilia are surrounded by an aqueous layer (periciliary fluid) secreted into the lumen of the airway passage by the active transport of chloride and the passive movement of water across the epithelium. The cilia make contact with the mucus floating on this aqueous layer, and via a unidirectional propelling motion provide for movement of mucus toward the glottis (see Pierson and Kacmarek, supra and Fauci, et al, supra). Mucus is produced by the epithelial goblet cells and submucosal gland cells and is secreted into the lumen of the airway after degranulation.
While mucus generally facilitates the dearance of inhaled partides or infectious agents, hypersecretion of mucus in the airways may cause progressive airway obstruction. In peripheral airways, cough is ineffective for clearing secretions. Furthermore, because of their small dimensions, small airways containing many goblet cells are especially vulnerable to airway plugging by mucus. Airway hypersecretion affects a substantial number of individuals; it is seen in a variety of pulmonary diseases, such as chronic bronchitis, acute asthma, cystic fibrosis, and bronchiectasis.
Hypersecretion of mucus is the major symptom in patents with chronic obstructive pulmonary disease (COPD) and defines the condition (i.e. chronic cough and sputum production). This condition alone affects 14 million Americans and can cause progressive disability and death. It has been estimated that asthma affects at least 4% of the U.S. population and accounts for at least 2000 deaths annually (Pierson and Kucmarek, supra). During an acute asthmatic event, the bronchial walls swell, mucus volume increases and bronchial smooth muscle contracts, resulting in airway narrowing. As a result of hypersecretion in acute asthma, extensive mucus plugging can be a major cause of morbidity and mortality.
Hypersecretion has also been implicated in cystic fibrosis, which is one of the most common, fatal, genetic diseases in the world. Cystic fibrosis is an autosomal recessive disease that causes the airway mucosal cell to become unresponsive to cyclic-AMP-dependent protein kinase activation of the membrane chloride ion channels (Pierson and Kacmarek, supra and Fauci, et al., supra). The subsequent electrolyte imbalance reduces the level of hydration of the airway mucus, thus resulting in highly viscous mucus in the lungs of an individual afflicted with cystic fibrosis. Hypersecretion obstructs the air passages of individuals with cystic fibrosis, further compromising lung function.
Other disease involving hypersecretion include chronic obstructive lung disorder (COPD). Oxidant stress plays an important role in the pathogenesis of COPD. Cigarette smoke, which generates oxygen free radicals, is strongly implicated in the pathogenesis. Neutrophils are often seen at site of inflammation in COPD, and interestingly, oxygen free radicals are known to be released by neutrophils during activation.
Mechanical intubation is often necessary in order to provide assisted ventilation to patients with various pulmonary diseases. A tube is introduced via the oropharanx and placed in the trachea. To prevent leaking of air around the endotracheal tube, a balloon is inflated around the tube in the lower trachea, which may abrade the epithelium and cause goblet cell metaplasia. Wounding of epithelium leads to repair processes, which can result in abundant mucus secretion. Such prolonged tracheal intubation in patients can lead to deleterious effects due to hypersecretion.
As a result of the high levels of mucus in the lungs of patients with hypersecretory pulmonary diseases, mucosal clearance is reduced. Pathological agents such as bacteria, e.g. Pseudomonas aeruginosa, often establish colonies within the mucus, resulting in frequent lung infection.
Classical modalities of treating individuals afflicted with airway hypersecretion include antibiotic therapy, bronchodilators (e.g., methylxanthines, sympathomimetics with strong β2 adrenergic stimulating properties, anticholinergics), use of systemic or inhaled corticosteroids, primarily in asthma, liquefaction of the mucus by oral administration of expectorants, e.g. guaifenesin, and aerosol delivery of “mucolytic” agents, e.g. water, hypertonic saline solution (see Harrison's, supra). A newer therapy for cystic fibrosis is the administration of DNAse to target the DNA-rich mucus or sputum (Shak, et al (1990) Proc. Nat. Acad. (USA) 87:9188-9192; Hubbard, R. C. et al. (1991) N. Engl. J. Med. 326:812). In addition, chest physical therapy consisting of percussion, vibration and drainage are also used to facilitate clearance of viscous mucus. Lung transplantation may be a final option for those with severe pulmonary impairment. Therefore, more efficacious or alternative therapy to target the mucosal secretions is needed. Specifically, there is a need for a specific modality that will reduce the formation of mucus secretions in the airways.